Electronics (e.g., transducers) are often designed with sealed enclosures (e.g., an airtight seal or housing) to protect the electronics from debris, water, and so forth. However, once these electronics are put into use, it is often the case that the airtight housing or seal leaks, because the airtight seals prevent the device's ability to equalize pressure, e.g., when there is a pressure differential. Pressure differentials are often caused by temperature changes. The temperature change can be internal, external or both. As the external pressure fluctuates (thus causing a pressure differential), the enclosure tries to equalize the internal pressure by drawing in air from the outside. If the housing is completely airtight, pressure builds up inside in the form of a positive or negative buildup. Positive buildup causes the housing to bloat, while negative buildup creates a vacuum. Either type of buildup leads to stress on the seal, which in turn compromises and damages it effectiveness. The compromised seals begin to allow water and contaminants to enter the housing, which can ultimately lead to electronic failure. (See, “The Unknown Problem with Airtight Enclosures,” www.ElectronicsProtectionMagazine.com).
To protect against these pressure differentials, more robust seals are often used. However, this solution will remedy the immediate leakage and contamination issues, but is a short-term fix that will ultimately fail because the fundamental problem of pressure differentials has not been addressed. The device is simply more airtight without having a solution for the root cause. (See, “The Unknown Problem with Airtight Enclosures,” www.ElectronicsProtectionMagazine.com).
As described in U.S. Pat. No. 7,439,616, a sealing ring is provide on the bottom of a packaged device that will seal the back volume during surface mounting to a user's board, as show in FIG. 1 (which is FIG. 3 in U.S. Pat. No. 7,439,616).
FIG. 2 is a bird's eye view of the bottom of package 10 shown in FIG. 1. In FIG. 2, substrate 14 of package 10 includes cavity 18. Sealing ring 22 surrounds cavity 18, making package 10 air-tight, when mounted to a board.